One special feature offered by mobile communications systems is group communication. Conventionally group communication has been available in trunked mobile communications systems, such as Professional Mobile Radio or Private Mobile Radio (PMR) systems, which are special radio systems primarily intended for professional and governmental users, such as the police, military forces and oil plants. However, the group communication is now becoming available also in public mobile communications systems. The term “group”, as used herein, refers to any logical group of three or more users for participating in the same group communication, e.g. a speech call. The same user may be a member of more than one communication group. Typically, the members of the communication group belong to the same organization, such as the police, the fire brigade, a private company, etc. Also, typically, the same organization has several separate groups, i.e. a set of groups. Even private persons might want to have talk groups, such as hobby groups, sport groups, etc.
Generally, in group voice communication with a “push-to-talk, release-to-listen” feature, a group call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his desire to speak, and the user equipment sends a service request to the network. Alternatively, a voice activity detector (VAD) can be used instead of the manual switch. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to all other active users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users can listen on the channel. When the user releases the PTT, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item, instead of reserving the resources for a “call”. One interesting advantage of push-to-talk communication, or more generally speech-item-by-speech-item communication, is a short call setup time, which also makes such speech communication attractive to several other types of users. U.S. Pat. No. 6,141,347 discloses a wireless communications system which uses multicast addressing and decentralized processing in group calls.
The group communication is now becoming available also in public mobile communications systems. New packet-based group voice and data services are being developed for cellular networks, especially in the GSM/GPRS/UMTS network evolution. In some approaches, the group communication service, and also one-to-one communication, is provided as a packet-based user or application level service so that the underlying communications system only provides the basic connections (i.e. IP connections) between the group communications applications in the user terminals and the group communication service. The group communication service can be provided by a group communication server system while the group client applications reside in the user equipment or terminals. Examples of this approach are disclosed in co-pending U.S. patent application Ser. Nos. 09/835,867; 09/903,871; 10/160,272; and 09/903,871; and in WO 02/085051. When this approach is employed for push-to-talk communication, the concept is also referred to as push-to-talk over a cellular (PoC) network.
Group communication has conventionally been based on predefined and rather static groups. In the first generation of PMR systems, information on group membership was stored only in mobile stations. In more recent systems, the information on group membership is stored both in the system and in the mobiles. The latter case provides many functional advantages (such as allowing the system to allocate resources only when actually required by a group member) but makes the dynamic regrouping technically more demanding and complex. The information in the system and the mobiles must match with one another. This is usually achieved by arranging the radio system to keep a master copy of the group information. This works well as long as it is enough to perform group management from a fixed terminal connected to the network infrastructure.
Various ways have existed for a long time to create new talk groups and/or to modify group membership more dynamically. A particular need has arisen to create and manage groups from mobiles. The groups should often be created spontaneously for temporary use. Such temporary and dynamic 3 groups are often called ad-hoc groups. Thus, for a good end-user experience regarding ad-hoc group call, a group call service in a mobile terminal and an infrastructure should offer an easy mechanism to form groups that have a short life and an easy-to-make group call.
Co-pending U.S. patent application Ser. No. 09/985,169 titled “A method for creating a dynamic talk group” and having the same assignee as the present application, discloses a method wherein group creation and membership management are handled at user level without interacting with the system. The role of the system is restricted to establishing a means for communicating with the group whenever there are users who have activated the communication group. A user creates a new dynamic group by defining a group definition message in his subscriber station. This group definition message uniquely identifies the new group in the system, and can be distributed to the intended group members using the common messaging facilities available in the specific communications system (such as SMS in a GSM network). The recipients of the group definition message can store the message in their subscriber stations for subsequent use. This prior art approach is, however, rather complicated for ad-hoc calls.